Before soldering I like to punch a few holes in the strips. Simply use an awl to punch a small hole in your strips using some soft wood underneath, flip it over repeat on the back side flatten your strip out with a hammer or whatever tool.
Now solder both sides and make sure it the solder sucks up the into the holes. Creates a very strong connection and more solder is in contact with the surface of the strip. I like to pool some solder on the backside. If the strips are doubled up which I usually do with the terminating ones, this is also a way to join them up if they extend beyond the cells.
I make the strips and the soldered leads up first then weld them down.